SiS in Circumstellar Shells

Abstract

We have observed the spectrum of SiS toward the Mira variable IRC+10216, including the J=16-15, J=15-14, J=14-13, J=13-12, J=12-11, J=7-6, J=6-5, J=5-4 and J=l-0 lines (Fig. 1), and made a detailed model to fit these lines, incorporating a radial SiS abundance gradient due to photodissociation by interstellar UV (Sahai, Wootten, and Clegg 1984). The strength of the high J lines requires a substantial excitation of the molecule by infrared radiation near the central star; the infrared also inverts the populations in the lower levels. Variations in the 13 urn infrared flux over the 644 day period of IRC+10216 produce observable changes in some of the SiS lines. Sharp cusps in the J=5-4 line wings, observed by Olofsson et al. (1982), are explained in terms of weak maser emission at maximum light. The model predicts a different shape for this line at minimum light, in excellent agreement with Olofsson’s (1985) recent observation. Line coincidences, involving 13 urn transitions of HCN and C₂H₂, which may affect the SiS vibration-rotation excitation, are identified.